About Boiler Design

Can you help me about boiler design. I want to know what is the best computer program for boiler design.
and I want detail step for mannual boiler design such as :
1.Assume boiler capacity
2.Find heat which system want
3.Find fuel consumption
4.etc........

Do you want to design a boiler or a heating system? If a system, you select the heating devices for the areas depending on the building's construction, THEN select a boiler which is adequate for that system. If you are trying to design a boiler, forget it, there are many which have already been designed which will be superior to anything you can construct.

I want to design boiler.
Now I'm working in company which maintanent and install boiler. we want to make a boiler for sale.
Today we can buit boiler from drawing ,we have, but when you want to sale boiler you must attach calculation note.
My problem is I don't know how to start calculation.